Photocatalysis involves the use of a semiconductor that when illuminated by light of sufficient energy produces highly reactive species that chemically attack molecules. Semiconductors and the photocatalysts into which they are incorporated are of particular interest to researchers and consumers, in large part because they may be used in pollution abatement applications, such as the treatment of gaseous, liquid and bacteriological matter. However, environmental applications are not their only use; for example, they may also be used to produce superhydrophilic surfaces for applications such as antifogging mirrors and self-cleaning windows.
Photocatalysts typically comprise one or more metal oxides, such as titanium dioxide, zinc oxide, tungsten oxide and/or zirconium oxide. Titanium dioxide, which is a particularly common photocatalyst, has for example, been used in applications, such as air-conditioning to remove toxic or malodorous substances, water purification and destruction of harmful bacteria.
Titanium dioxide, also commonly referred to as titania or titanium oxide, may exist in one of three crystalline forms, rutile, anatase and brookite or mixtures of these forms, as well as in the non-crystalline form of titanium hydroxide. Commercially, titanium dioxide may be used in both pigmentary applications, e.g., coatings, plastics and paints, as well as in non-pigmentary applications, e.g., ultraviolet light absorbing materials such as sunscreens and photocatalysts. However, of the aforementioned forms, only the crystalline forms exhibit photocatalytic activity, and of the crystalline forms, only anatase and rutile have potential commercial applicability as photocatalysts.
Photocatalysts function by absorbing light of sufficient energy to promote an electron of a semiconductor from the valence band to the conduction band. This energy is referred to as the band gap energy, and is typically quoted in electron-volts (eV), or by the wavelength (in nanometers or nm) of a photon that possesses this band gap energy. For example, rutile titanium dioxide has a band gap energy of 3.0 eV, which corresponds to approximately 413 nm, and anatase titanium dioxide has a band gap of 3.2 eV, which corresponds to approximately 388 nm.
Within the field of photocatalysis, anatase titanium dioxide has established a reputation as generally superior to rutile titanium dioxide, though no fundamental reason for this difference has been identified. By contrast, the primary commercial applications of rutile titanium dioxide have been as white pigments due to rutile titanium dioxide's high refractive index, and secondarily as ultraviolet absorbers in sunscreens, coatings and plastics applications.
When a titanium dioxide crystal absorbs a photon of sufficient energy, an electron is promoted to the conduction band, and a positive hole (h+) is generated in the valence band. This “electron-hole pair” generates highly oxidizing hydroxyl and superoxide radicals at the crystal surface that are capable of oxidizing practically all organic matter to CO2 and H2O. This complete oxidation is referred to as “mineralization.”
The early applications of titanium dioxide in photocatalysts used nearly exclusively ultraviolet light sources as is required by the band gap energies of anatase titania. Ultraviolet (UV) light refers to light of a wavelength below about 400 μm. By contrast, light from the visible spectrum refers to light of a wavelength from about 400 nm to about 700 nm.
Recently, there has been attention given to finding ways in which the photocatalytic activity of titania can be expanded into the visible spectrum, in part because UV light constitutes only a relatively small fraction of sunlight and in part because indoor lighting is typically very low in UV intensity. If a larger proportion of the electromagnetic spectrum could be used, then a more efficient catalytic performance could be achieved and a larger number of potential applications could be realized.
Attempts to utilize visible light have included doping photocatalysts with elements such as chromium, reducing titanium dioxide and doping with nitrogen. Unfortunately, all of these techniques suffer from the drawback of generating a colored product, which is to be expected if some of the visible light is absorbed; Although the generation of a colored product may be satisfactory from a technical perspective, due to aesthetic constraints, a colored product tends to limit commercial applicability. A more desirable product would be a white product that more efficiently utilizes available near-UV light, i.e., visible light, than the current commercial anatase products utilize.
As noted above, rutile titanium dioxide has a band gap that utilizes a larger proportion of the available spectrum and would seem to be an ideal choice for a photocatalyst that operates under low levels of UV illumination, without compromising the aesthetics of color. Historically, when rutile titanium dioxide has been considered applicable as a photocatalyst, it has been in a state in which the rutile crystals are “metallized,” that is, containing small islands of metals such as platinum, rhodium, gold and silver on the rutile crystals. Unfortunately, such catalysts are prohibitively costly.
Thus, there is a need to develop a useful and economical rutile titanium dioxide photocatalyst that can be activated by visible light. The present invention addresses this need.